1. Field of the Invention
The present invention relates to a supply circuit for an electronic circuit connected to a switched mode power supply (SMPS) converter operating at low output voltage. In particular, the invention relates to a supply circuit for an integrated circuit operating as a switch of an SMPS converter having a circuit topology of the buck (step down or forward) type or of the buck-boost (step up/down or flyback) type.
2. Description of the Related Art
As is known, SMPS converters having a circuit topology of the buck type or the buck-boost type use inductive components, the charging and discharging of which are controlled by switches that operate switching between a saturation condition and an inhibition condition (ON/OFF).
For a more detailed treatment of SMPS converters of the type referred to above, see, for example, J. G. Kassakian, M. F. Schlecht, G. C. Verghese xe2x80x9cPrinciples of Power Electronics,xe2x80x9d Addison Wesley.
At present, the aforesaid supply circuits are connected to the output of the converter and are able to operate only when the output voltage of the converter has a value greater than a preset value (8-9 V). For a better understanding, see for example the supply circuit provided in a standard buck converter illustrated in FIG. 1.
In FIG. 1, a buck converter comprises an integrated circuit 2, forming a controlled power switch, an inductor 3, a first diode 4, a first capacitor 5, and a supply circuit 6. The converter 1 moreover has a first input pin 10 receiving an input voltage Vin, a second input pin 11 connected to a ground line 12, a first output pin 13 supplying an output voltage Vo, which is positive with respect to the ground line 12 and has a value lower than that of the input voltage Vin, and a second output pin 14, connected to the ground line 12.
In detail, the integrated circuit 2, typically formed by an NMOS power transistor and a control circuit, has a first terminal (drain) connected to the first input pin 10, a second terminal (source) connected to a first intermediate node 18, and a supply input 19 receiving a supply voltage Vcc. The inductor 3 has a first terminal connected to the first intermediate node 18 and a second terminal directly connected to the first output pin 13.
The first diode 4 has its cathode connected to the first intermediate node 18 and its anode connected to the ground line 12.
The first capacitor 5 has a first terminal connected to the first output pin 13 and a second terminal connected to the second output pin 14.
The supply circuit comprises a second diode 20 and a second capacitor 21. The second diode 20 has its anode connected to the first output pin 13 and its cathode connected to the supply input 19 of the integrated circuit 2. The second capacitor 21 is connected between the first intermediate node 18 and the supply input 19 of the integrated circuit 2.
In a known way, the integrated circuit 2 performs a power conversion between the first input pin 10 and the first intermediate node 18. The voltage on the first intermediate node 18 is filtered by the inductor 3 and by the first capacitor 5. The first diode 4 enables recirculation of the current of the inductor 3 when the integrated circuit 2 opens, disconnecting the first intermediate node 18 from the first input pin 10.
The second diode 20 connects the first output pin 13 to the supply input 19 of the integrated circuit 2 and discharges the second capacitor 21 during opening of the integrated circuit 2. The second capacitor 21 filters the output voltage Vo and stabilizes it.
The supply circuit 6 of FIG. 1 can be used if the output voltage Vo is higher than the supply voltage Vcc of the integrated circuit 2, since
Vcc=Voxe2x88x92VD 
where VD is the voltage drop on the second diode 20.
Consequently, the supply circuit 6 of FIG. 1 is limited in its application.
In order to extend the range of output voltage of the converter 1 of FIG. 1, one known solution is to provide a double winding on the inductor 3, as shown in FIG. 2, wherein a second winding 3a is connected between the first intermediate node 18 and the anode of a second diode 20xe2x80x2, so as to take and supply to the integrated circuit 2 the voltage present on the first intermediate node 18, which is higher than the voltage present on the first output pin 13 when the integrated circuit 2 is closed.
The above solution is advantageous only in insulated converters, wherein a high-frequency transformer is already available, as in the case, for example, of a flyback converter provided with transformer, in which the introduction of an additional winding does not affect costs very much.
Instead, this solution is not economically advantageous in non-insulated converters using a standard inductor insulated in lacquer.
The disclosed embodiments of the present invention provide a supply circuit for an electronic circuit connected to an SMPS converter operating at a low output voltage that does not present the drawbacks previously described.
According to the embodiments of the present invention, there is provided an SMPS converter having a first input receiving an input voltage, an output supplying an output voltage, a controlled switch connected between said first input and an intermediate node, a first component connected between said intermediate node and said output, a second component connected between said intermediate node and a reference-potential line, one of said first component and said second component comprising an inductive element, and another of said first and second components comprising a unidirectional current-conducting element, and a supply circuit connected to said inductive element and having an output pin that supplies a supply voltage, the supply circuit having an energy taking element connected to said inductive element, an energy accumulation element connected to said energy taking element and storing an electric voltage, and a voltage transfer element connected between said energy accumulation element and said output pin of said supply circuit.